1. Field of the Invention
This present invention pertains to the production of flour from cereal grains and to the production of high-starch feed stocks for conversion to bio-fuels, and more particularly to a bran-finishing process that uses a milling machine for detaching flour from bran using compressive and/or abrasive forces.
2. Description of the Related Art
Cereal grains are consumed throughout the world as a staple food, which often serves as a primary source for carbohydrates and also as a feedstock for the production of bio-fuels such as ethanol. Cereal grains have an embryo or germ surrounded by endosperm that is in turn surrounded by bran layers. The endosperm has a high starch content, which makes cereal grains a good source of food for humans. Cereal grains include rye, barley, wheat, which includes durum or durum wheat, hard wheat, and soft wheat, and triticale, which is a hybrid of rye and wheat.
FIG. 1 shows a kernel 10 of wheat in cross-section as an example of a cereal grain. Kernel 10 comprises an outer bran coat 12 surrounding and protecting an inner portion of endosperm 14, which surrounds or is adjacent to an embryo or germ portion 16. FIG. 1A shows the cross-section of the outer bran coat 12 in greater detail. Outer bran coat 12 is adjacent to endosperm 14. Outer bran coat 12 has an innermost layer of aleurone 18 in contact with endosperm 14. A seed coat 20 of nucellar tissue covers the aleurone layer 18. A layer of testa 22 covers the seed coat nucellar tissue 20, and pericarp 24 covers the testa 22. U.S. Pat. No. 5,082,680, issued to Tkac, and U.S. Pat. No. 5,846,591, issued to Satake et al., which are incorporated by reference, provide additional detail on the composition of a grain of wheat. U.S. Pat. No. 5,211,982, issued to Wellman and incorporated by reference, provides background information on milling wheat and producing a milled product that contains aleurone cell wall fragments.
Traditional methods of cereal milling involve conditioning the kernel by increasing its moisture content and then subjecting the kernel to several successive stages of impact or crushing actions intended to first dislodge the germ and break the whole kernel into several pieces, then further reduce these pieces into smaller particles until endosperm of the desired particle size is obtained. During these impact and crushing actions, bran particles tend to stay whole, while germ tends to flatten, producing relatively larger particles, and the endosperm is scraped from the bran particles and tends to shatter, producing relatively smaller particles. Between the impact or crushing stages, sifting and/or density separation is employed to isolate the bran and the germ from the endosperm.
Once the germ and the bran have been isolated from the endosperm, the germ can be separated from the bran using rollermilling, sifting and aspiration. The bran is then subjected to impact in an attempt to dislodge any endosperm particles that still cling to the bran, and these endosperm particles are then removed from the bran by screening. Inherent inefficiencies in this process for detaching endosperm from bran and the machinery used in recovering the endosperm from the bran limit the yield of endosperm that can be used for the production of flour or bio-fuel feedstock. A further drawback is that the aleurone, which is a largely colorless and nutrient-rich layer on the surface of the endosperm, is largely lost to the bran using conventional milling technologies. Aleurone is valued for the nutrients and dietary fiber that it contains, but aleurone adheres tightly to the bran and stays with the bran.
In conventional milling of wheat, as much endosperm and aleurone has been recovered from bran as practical by passing grains of wheat through a pair of rollers, which crush the grains of wheat, followed by sifting for particle size classification. This is referred to as a break, and larger particles are passed through another set of rollers, which is referred to as a second break, followed by sifting for size classification. This process can be repeated for as many breaks as economically practical, and a typical mill may employ four, five or six breaks. The process continues into a bran finishing step and a shorts finishing step.
FIG. 2 illustrates a conventional prior art bran finishing machine 30, which is shown as a partial cross-section of an end view of a side elevation. Bran finishing machine 30 has a housing 32, which is supported by legs (not shown). Housing 32 has a length defined by opposing ends, and a shaft-support with a bearing (not shown) is located on each end. A shaft 34 extends essentially the length of the housing 32 and is received in the shaft supports. A motor (not shown) rotates shaft 34 using a belt (not shown). A plurality of hubs 36 is received on and fixed to shaft 34, although only one hub 36 is shown in the cross-section. Each hub 36 has spokes 36a, 36b, 36cf, 36d and 36e that extend radially outwardly from shaft 34. Beaters 38a, 38b, 38c, 38d and 38e are received on spokes 36a, 36b, 36cf, 36d and 36e, respectively. The spokes shall be referred to collectively as spokes 36, and the beaters shall be referred to collectively as beaters 38. The beaters 38 are plates that extend essentially the length of the housing 32. Housing 32 has brackets 32a and 32b, and a cover 40 having brackets 40a and 40b is bolted to brackets 32a and 32b, respectively. Cover 40 extends the length of the housing 32 and is open along an upper longitudinal portion, which gives cover 40 a trough shape. In the transverse cross-section shown in FIG. 2, cover 40 has a circular shape and is enclosed along a lower portion of about 220 degrees, leaving an open upper portion of about 140 degrees. An upper portion 32c of housing 32, housing sidewall 32d and 32e (not shown), bracket flanges 32a and 32b and cover 40 define a milling chamber 42, which is an enclosed space. Housing 32 has a discharge hopper 32f below cover 40. Cover 40 is perforated, which provides a plurality of cover holes (not shown) that extend radially through cover 40. The cover holes provide pathways for flour to pass from inside cover 40 to the inside of discharge hopper 32f. An inlet opening 32g is referred to as a loading spout and provides a pathway for feeding a cereal grain, such as wheat, into milling chamber 42. An outlet opening (not shown) through housing 32 on the end opposite inlet opening 32g provides a pathway for discharging bran from milling chamber 42.
In operating bran finisher 30, bran is fed continuously into milling chamber 42 through inlet opening 32g. The bran has aleurone attached to an inside layer of the bran and endosperm attached to the aleurone and clinging to the bran. Milling chamber 42 remains partially filled. Shaft 34 is rotated by the motor, which causes beaters 38 to pass through the bran. Beaters 38 hit, strike and collide with the bran and thus impact the bran. Beaters 38 are shaped to push the bran toward the outlet opening. As beaters 38 impact the bran, a portion of the endosperm is removed from the bran, and that portion of the endosperm passes through the cover holes and into the discharge hopper, where it is recovered for further processing into flour. The bran is moved along by the beaters 38 to the outlet opening. A plurality of diverting paddles 44 can be rotated with an adjusting screw 44a for controlling the length of time that the bran is in the milling chamber 42. Although a portion of the endosperm can be removed from the bran by impact forces applied by the bran finisher 30, the aleurone layer tends generally to remain adhered to the bran. Consequently, the aleurone layer, which has good nutritional value, is usually lost with the bran and is not recovered for use in flour.
It is desirable to recover the nutrient-rich aleurone layer from the bran for improving flour yield and the nutritional value of the flour, and processes have been developed for recovering aleurone from bran. U.S. Pat. No. 4,746,073, issued to Stone et al. and incorporated by reference, describes a process for recovering aleurone from wheat bran, which includes hammer-milling the wheat bran, which is believed to detach the aleurone from the bran and to break the bran and aleurone into small particles. The mixture of bran and aleurone particles are classified to achieve a desired particle size range. Particles of the desired particle size range are electrostatically charged and then fractionated by the differential of their electrical charges, thereby separating the aleurone from the bran for recovering some portion of the aleurone.
U.S. Patent Application Pub. No. 2003/0175384, which lists Bohm et al. as inventors and which is incorporated by reference, is directed to extracting aleurone from bran. A wet method is described that uses enzymes to weaken the adhesion of aleurone to bran. A dry method is described that uses a rolling mill, a centrifugal impact mill and/or a jet mill for detaching aleurone from bran and for grinding and/or breaking the aleurone and bran into a mixture of small particles. The mixture is separated into its aleurone and bran components by air classification and sifting and by grading or sorting in an electrical field, all of which can be repeated to obtain a desired level of enrichment of aleurone cells.
U.S. Patent Application Pub. No. 2006/0177529, which lists Laux et al. as inventors, describes a process for recovering aleurone from bran that has aleurone components and bran components. The aleurone components are detached from the non-aleurone components using mechanical-abrasive means or biological-enzymatic means as described in U.S. Patent Application Pub. No. 2003/0175384, which forms a mixture composed of aleurone and non-aleurone components. The aleurone components are separated and recovered from the mixture using electrostatic sorting. Water is added to the aleurone components to provide a moisture content of 10-20 wt %, followed by superfine milling using a grinding roll mill in which rolls are pressed together while revolving at different speeds.
In these prior efforts to recover aleurone, which is adhered to bran initially, the aleurone and bran are believed to have been generally broken down into a mixture of fine particles. Separation of the aleurone components from the non-aleurone components in the mixture required significant capital and operating expenditures for equipment such as electrostatic chargers, air classifiers, sifters, sieves and sorters.